Difference in proton radii of mirror nuclei as a possible surrogate for the neutron skin

It has recently been suggested that differences in the charge radii of mirror nuclei are proportional to the neutron-skin thickness of neutron-rich nuclei and to the slope of the symmetry energy L [Brown, Phys. Rev. Lett. 119, 122502 (2017)]. The determination of the neutron skin has important implications for nuclear physics and astrophysics. Although the use of electroweak probes provides a largely model-independent determination of the neutron skin, the experimental challenges are enormous. Thus, the possibility that differences in the charge radii of mirror nuclei may be used as a surrogate for the neutron skin is a welcome alternative. To test the validity of this assumption we perform calculations based on a set of relativistic energy density functionals that span a wide region of values of L. Our results confirm that the difference in charge radii between various neutron-deficient nickel isotopes and their corresponding mirror nuclei is indeed strongly correlated to both the neutron-skin thickness and L. Moreover, given that various neutron-star properties are also sensitive to L, a data-to-data relation emerges between the difference in charge radii of mirror nuclei and the radius of low-mass neutron stars.